This invention relates generally to processes for forming paperboard products and to the products formed by such processes. More particularly, this invention relates to a method of making disposable paperboard containers with textured coatings and to the texture-coated containers formed by that method. This invention also relates to coatings having superior bulk and insulation properties.
In addition, this invention relates to an improved paperboard, to improved shaped paperboard products, and to methods of making such paperboard and shaped paperboard products, including heat insulating paperboard containers, such as cups, having as their wall surface a foamed layer of thermoplastic film. More particularly, this invention is also directed to an improved bulk-enhanced paperboard, to methods of making such a paperboard, and to shaped paperboard products made from such paperboard.
In one aspect of the present invention, insulating and/or textured coatings having a high coefficient of friction are printed on a paperboard. The printing of the coating is an efficient, precise process allowing as little as about ten percent of the container surface to be coated to achieve beneficial insulation and handling properties. These containers are particularly suitable for use as hot drink containers, since only a small portion of the outer surface of the container has to be printed. Foamed polyolefin insulated coating cannot be printed onto the surface of the paperboard and, consequently, the whole side of the paperboard has to be coated. The coated containers of this invention have superior insulation and bulk properties and have greater inherent cost advantages over the prior art foamed polyolefin extrusion coated containers Furthermore, the registered, texture coated containers of the present invention exhibit excellent printing clarity and accuracy which cannot be obtained when coatings are prepared from foamed polyolefins.
Disposable paper containers, such as plates, trays, bowls, airline meal containers and cafeteria containers, are commonly produced by pressing flat paperboard blanks into the desired shape between appropriately shaped and heated forming dies. Various protective coatings are typically applied to the blanks before forming to make the resulting paperboard containers moisture-resistant, grease-resistant, more readily printable, etc. Often, printing is also applied to the top surface for decoration. A large number of paper products are produced by this method every year. These products come in many different shapes and sizes, including round, rectangular, and polygonal. Many such containers, including for example airline meal containers, have a number of independent compartments separated by upstanding ridges formed in the inner areas of the containers.
When a container is made by pressing a flat paperboard blank, the blank should contain enough moisture to make the cellulosic fibers in the blank sufficiently plastic to permit it to be formed into the desired three-dimensional container shape. During the pressing operation, most of this moisture escapes from the uncoated bottom surface of the blank as water vapor. Suitable methods of producing paperboard containers from moistened paperboard blanks are generally described in U.S. Pat. Nos. 4,721,499 and 4,721,500, among others.
Many people prefer disposable containers which, when handled, produce a sense of bulkiness and grippability at least suggestive of the more substantial non-disposable containers which they replace. While a sense of bulkiness may be provided to some extent in styrofoam and thick pulp-molded containers, such containers suffer a number of drawbacks For example, unlike pressed paperboard containers, styrofoam containers are often brittle and they are environmentally unfriendly because they are not biodegradable. Also, styrofoam containers are not cut-resistant and it is difficult to apply printing to the surface of styrofoam containers. Additionally, because of their bulkiness, styrofoam containers take up large amounts of shelf space and are costly to ship. Pulp-molded containers similarly are not cut-resistant and have poor printability characteristics. Additionally, pulp-molded containers typically have weak bottoms. Pressed paperboard containers, however, are cut-resistant, readily printable, strong in all areas, and are far less bulky than styrofoam or pulp-molded containers.
The present invention is an improvement in pressed paperboard containers. In the present invention, environmentally friendly disposable paperboard containers are formed. By printing an insulating and/or textured coating on as little as ten percent of one surface of the paperboard, insulating and/or textured containers are formed which give users handling them a sense of bulkiness and grippability. These new containers rely on efficient processes of press-forming paperboard blanks. The resulting product, which consists primarily of cellulosic material, is nearly entirely biodegradable. Additionally, the product of the present invention may withstand normal microwave conditions without any significant change in caliper, may have substantially better thermal resistance when compared to prior disposable paperboard containers made without such an insulating and/or textured coating, and may tend to stay put when resting on a smooth surface due to the coefficient of friction of the textured coating. It should be noted that prior art polyolefin foamed coatings cannot be pattern applied, and therefore have to cover the whole side of the board.
The data shown in FIGS. 9A and 9B demonstrates that conventional paper plates have a coefficient of kinetic friction of about 0.18, plastic plates have a coefficient of kinetic friction of about 0.2, and foam plates have a kinetic coefficient of friction of slightly under 0.2. The coefficient of kinetic friction of the textured plates of this invention may have values of from about 0.61 to 1.4 and up to about 2.0 and more. Thus, the coefficient of kinetic friction of the texturized plates of this invention is up to at least about seven times greater than for conventional paper plates. Accordingly, the suitable coefficient of kinetic friction for the texturized containers of the present invention may be from about 0.22 to at least about 2.0. In one embodiment, the kinetic coefficient of friction is from about 0.4 to about 0.9. In another embodiment, the kinetic coefficient of friction is from about 0.5 to about 0.7.
The data shown in FIGS. 9A and 9B also demonstrates that conventional paper plates and plastic plates have a static coefficient of friction of 0.19. For foam plates the coefficient of static friction is 0.2. The static coefficient of friction of containers of the present invention is from about 0.2 to 2.0. In one embodiment, the coefficient of static friction is from about 0.4 to about 1.5. In another embodiment, the coefficient of static friction is from about 0.4 to about 1.0. Thus, the static coefficient of friction of the paperboard of the present invention is up to at least about ten times greater than for conventional plates.
The texture coated cellulosic paperboard must reconcile several conflicting properties to be useful for the manufacture of plates, cups, bowls, canisters, French fry sleeves, hamburger clam shells, rectangular take-out containers, and related articles of manufacture. The coated paperboard should have improved thermal resistance, improved formability, and, to improve economics, the whole board need not be covered with the coating. All of the conventional paperboards can be utilized; but for enhanced insulation properties, the fiber weight (hereinafter “w”) of the paperboard should be at least about forty pounds for each three thousand square foot ream. However, for some applications, enhanced properties are achieved for paperboards having a fiber weight of about 10 pounds or less for each three thousand square foot ream. Fiber weight is the weight of fiber in pounds for each three thousand square foot ream. The fiber weight is measured at standard TAPPI conditions which provide that the measurements take place at a fifty percent relative humidity at seventy degrees Fahrenheit. In general, the fiber weight of a 3000 square foot ream is equal to the basis weight of such a ream minus the weight of any coating and/or size press. The fiber mat density of the paperboard utilized in the manufacture of textured containers should be in the range of from at least about 3 to at least about 9 pounds per 3000 square foot ream at a thickness of 0.001 inches. The fiber mat density of the paperboard can be greater that 9 pounds per 3000 square foot ream at a thickness of 0.001 inches. In one embodiment, the fiber mat density is in the range of at least about 4.5 to at least about 8.3 pounds per 3000 square foot ream at a fiberboard thickness of 0.001 inch.
In one embodiment, for a the board at a fiber mat density of 3, 4.5, 6.5, 7, 8,3, and 9 pounds per 3000 square foot ream at a thickness of 0.001 inch, the GM Taber stiffness may be at least about 0.00716w2.63 grams-centimeter/fiber mat density1.63 The GM tensile stiffness may be at least about 1890+24.2w pounds per inch. In another embodiment, the GM Taber stiffness value for paperboards having the fiber mat density given above may be at least about 0.00501w2.63 grams-centimeter/fiber mat density1.63. The GM tensile stiffness may be at least about 1323+24.2w pounds per inch. In yet another embodiment, the GM Taber stiffness may be at least about 0.00246w2.63 grams-centimeter/fiber mat density1.63. The GM tensile stiffness may be at least about 615+13.18w pounds per inch. The GM Taber stiffness values listed are desired to facilitate the bending of the paperboard into the aforementioned articles of manufacture and to provide these articles with greater rigidity. Likewise, the GM Taber stiffness and GM tensile stiffness prevent the plates, cups, and other articles of manufacture from collapsing when used by the consumer. The articles of manufacture can suitably be prepared from either one-ply or multi-ply paperboard, as disclosed herein. The GM tensile and GM Taber values for the web and one-ply board may be the same. For multi-ply board the overall paperboard GM Taber stiffness and GM tensile stiffness may be the same as for a one-ply paperboard. The aforementioned combination of GM Taber stiffness and GM tensile stiffness provide a paperboard which can readily be converted to useful high quality textured or insulation coated cups, plates, compartmented plates, bowls, canisters, French fry sleeves, hamburger clam shells, rectangular take-out containers, food buckets, and other consumer products and other useful articles of manufacture which have the outer surface partially texture coated and/or insulation coated.
Suitable one-ply and multi-ply paperboards may comprise (a) predominantly cellulosic fibers, (b) bulk and porosity enhancing additives interspersed with the cellulosic fibers in a controlled distribution throughout the thickness of the paperboard, and (c) size press applied binder coating, optionally including a pigment, adjacent both surfaces of the paperboard and penetrating into the board to a controlled extent. In one embodiment, the amount of size press applied is at least about one pound for each three thousand square foot ream of paperboard having a fiber mat density of about 3 to below about 9 pounds per 3000 square foot ream at a board thickness of 0.001 inches. For boards having a fiber mat density of 9 or greater per 3000 square foot ream at a board thickness of 0.001 inches, the amount of size press applied may be at least about six pounds for each three thousand square foot ream.
Prior art bulk-enhanced paper products, such as those disclosed in U.S. Pat. Nos. 3,941,634 and 3,293,114, resulting from the addition of expandable microspheres and other bulk enhancing additives and methods for making such paper suffer from a number of drawbacks. For example, one persistent problem in such papers is poor retention of the expandable microspheres or other bulk enhancing additives on the embryonic paper web made in the course of manufacturing the paperboard. This poor retention results in relatively low bulk enhancement of the resulting paperboard per unit weight of bulk enhancing additive added, making the enhancement process unnecessarily costly. A further problem resulting from the poor retention of microspheres and other bulk enhancers experienced in prior art bulk enhancement methods is fouling of the papermaking apparatus with unretained microspheres and other bulk enhancing additives.
A related problem associated with the addition of microspheres and other bulk enhancing additives in the papermaking process is their uneven distribution within the resulting paperboard. Paperboards prepared using prior art enhancement techniques have exhibited a decided asymmetry, with microspheres and other bulk enhancing additives migrating to one of the outer surfaces of the paper web and causing undesired roughness in the surface of the finished paper and hence interference with the smooth and efficient operation of the papermaking apparatus.
The void volume provided by the microspheres reduces the rate of thermal transfer within the paper, which is desirable in many applications. However, the asymmetric distribution of microspheres experienced in the prior art produces uneven thermal insulating characteristics.
In addition, prior art techniques have not created a satisfactory bulk-enhanced paperboard. Prior art products tend to have low thermal insulative properties. The excessive concentration of microspheres at the paper surface creates dusting, which interferes with the operation of printing presses in which the paperboard is used. The printability of the paperboard itself, that is, the satisfactory retention of printed matter on the paperboard, is also adversely affected by such dusting.
Prior art attempts at addressing the above and other drawbacks and disadvantages of paper containing microspheres and other bulk enhancing additives have been unsatisfactory and have had their own drawbacks and disadvantages. For example, in U.S. Pat. No. 3,941,634, Nisser attempts to address the inadequate retention and non-uniform distribution of microspheres by sandwiching the microspheres between two paper webs formed on two wire screens. The introduction of the second paper web adds complexity and expense to the papermaking process. Furthermore, the Nisser process generally does not optimize thermal insulation characteristics because it does not produce a sufficiently even distribution of microspheres within the resulting paper. The same problems are encountered in U.S. Pat. No. 3,293,114 and make the use of current bulk-enhanced papers in thermal insulation applications problematic.
Another attempted solution to the above and other drawbacks and disadvantages of paper containing microspheres has been to employ a surface sizing formulation to “bury” the microspheres which would otherwise be found on the outer surface of the resulting paper. See for example, Development of a Unique Lightweight Paper, by George Treier, TAPPI Vol. 55, No. 5, May 1972. This approach, again, has failed to achieve the desired distribution and retention of microspheres, as well as other desirable paper characteristics. In addition to the expensive film forming materials described in the George Treier article, the Treier process increases the complexity and cost of manufacturing paperboard.
The process of making cups, plates, bowls, canisters, French fry sleeves, hamburger clam shells, rectangular take-out containers, food buckets, and other shaped paper articles by deforming bulk-enhanced paperboards of the prior art to create the desired shapes also suffers from various drawbacks and disadvantages. Such paperboard is generally rendered substantially less deformable after being bulk-enhanced by the additions of microspheres. This reduced deformability interferes particularly with top curl forming in rolled brim containers made from bulk-enhanced paperboard. It also interferes with the drawing of cups, plates, bowls, canisters, French fry sleeves, hamburger clam shells, rectangular take-out containers, and food buckets, the reduced deformability in forming dies, and all other applications requiring deformation of bulk-enhanced paper generally and bulk-enhanced paperboard in particular.
Accordingly, there is a need for an improved, bulk-enhanced paperboard which retains a higher percentage of added bulk enhancers in the center layer of the board than has heretofore been achieved. In the paperboard of the present invention, the distribution of the bulk and porosity enhancing additive may be controlled so that at least about twenty percent of the additive is distributed in the central layer and not more than about 75 percent of the additive is distributed on the periphery of the paperboard with no periphery having more than twice the percent of the additive distributed in the central layer of the paperboard.
The present invention provides a bulk-enhanced cellulosic paperboard which, at a fiber mat density of 3, 4.5, 6.5, 7, 8.3, and 9 pounds per 3000 square foot ream at a fiberboard thickness of 0.001 inches, may have a GM Taber stiffness of at least about 0.00716w2.63 grams-centimeter/fiber mat density1.63. The GM tensile stiffness may be at least about 1890+24.2w pounds per inch. In one embodiment, the GM Taber stiffness for the paperboard of this invention having a fiber mat density of 3, 4.5, 6.5, 7, 8.3, and 9 pounds per 3000 square foot ream at a fiberboard thickness of 0.001 inches may be at least about 0.00501w2.63 grams-centimeter/fiber mat density1.63. The GM tensile stiffness may be at least about 1323+24.2w pounds per inch. In yet another embodiment, the GM Taber stiffness may be at least about 0.00246w2.63 grams-centimeter/fiber mat density1.63. The GM tensile stiffness may be at least about 615+13.18w pounds per inch. At a fiber mat density of 3, 4.5, 6.5, 7, and 8.3 pounds per 3000 square foot ream at a fiberboard thickness of 0.001 inches, the GM Taber stiffness may be at least about 0.00120w2.63 grams-centimeter, at least about 0.00062w2.63 grams-centimeter, at least about 0.00034w2.63 grams-centimeter, at least about 0.00030w2.63 grams-centimeter, and at least about 0.00023w2.63 grams-centimeter, respectively. The GM Taber stiffness may be at least about 1890+24.2w pounds per inch. In another embodiment, the GM Taber stiffness values for a fiber mat density of 3, 4.5, 6.5, 7, and 8.3 pounds per 3000 square foot ream at a fiberboard thickness of 0.001 inches, may be at least about 0.00084w2.63 grams-centimeter, at least about 0.00043w2.63 grams-centimeter, at least about 0.00024w2.63 grams-centimeter, at least about 0.00021 w2.63 grams-centimeter, and at least about 0.00016w2.63 grams-centimeter, respectively. The GM tensile stiffness value may be at least about 1323+24.2w pounds per inch.
There is a further need for an efficient, economical method of ensuring a better distribution of bulk additives in paperboard intended for use in shaping containers and other products in which good insulating characteristics and deformability are desired.
There is a further need for bulk-enhanced paperboard whose manufacture does not cause fouling by unretained microspheres and which operates on conventional papermaking machinery without causing dryer sticking problems and without interfering with printing operations to which the paperboard may be exposed.